In a Long Term Evolution (LTE) Release 8/9/10 (Rel-8/9/10) system, Physical Downlink Control Channels (PDCCHs) are transmitted in first N Orthogonal Frequency Division Multiplexing (OFDM) symbols of each radio sub-frame. N may take the values of 1, 2, 3 and 4, and N=4 is only allowed to occur in a system with the system bandwidth of 1.4 MHz. The first N OFDM symbols of the radio frame are also referred to a “legacy PDCCH region”.
In the LTE Rel-8/9/10 system, the legacy PDCCH region is logically divided into Control Channel Elements (CCEs). A CCE is composed of nine Resource Element Groups (REGs). The CCE is mapped to the REGs by being mapped to the entire bandwidth range while the REGs are interleaved. An REG is composed of four Resource Elements (REs) duplicated in the time domain and adjacent in the frequency domain. There is no common reference symbol transmitted in any of the REs of which the REG is composed.
Downlink Control Information (DCI) is transmitted in a unit of CCE. A piece of DCI for a User Equipment (UE) (also referred to a terminal) can be transmitted in M logically consecutive CCEs. M in the LTE system may take the value of 1, 2, 4 or 8 and also can be referred to as a CCE aggregation level. The UE performs blind PDCCH detection in the legacy PDCCH region to search for a PDCCH transmitted thereto. The so-called blind detection refers to that a decoding attempt is made for different DCI formats and CCE aggregation levels using a Radio Network Temporary Identity (RNTI) of the UE, and if decoding is correct, then DCI for the UE is received. The UE performs blind detection in the legacy PDCCH region of each downlink sub-frame in a Discontinuous Reception (DRX) state to search for a PDCCH.
In order to improve the performance of a Long Term Evolution-Advanced (LTE-A) system and extend the capacity of PDCCHs, an Enhanced Physical Downlink Control Channel (E-PDCCH) has been introduced to the Rel-11. E-PDCCHs are transmitted in the existing region of Physical Downlink Shared Channels (PDSCHs).
In a LTE Time Division Duplex (TDD) system, a 10 ms radio frame includes ten sub-frames with the length of 1 ms. Transmission directions of some of the sub-frames can be configured, particularly as depicted in Table 1, where D represents a downlink sub-frame, U represents an uplink sub-frame, and S represents a special sub-frame. The downlink sub-frame and the uplink sub-frame are also referred to as normal sub-frames. A special sub-frame has a length of 1 ms and is composed of three time slots which are a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP) and an Uplink Pilot Time Slot (UpPTS), and the lengths of these three time slots can be configured. Special sub-frame configurations supported in the LTE Rel-11 are as depicted in Table 2. Transmission of E-PDCCHs also needs to be supported in the downlink component DwPTS in the special sub-frame. As depicted in Table 2, taking a normal Cyclic Prefix (normal CP) as an example, the DwPTS can include OFDM symbols, the number of which may be one of {3, 6, 9, 10, 11, 12}, whereas the normal downlink sub-frame includes 14 OFDM symbols. As a result, the number of REs really available in each Physical Resource Block (PRB) pair in the DwPTS may be greatly lowered as compared with the normal sub-frame.
TABLE 1Uplink-downlink configurations in LTE TDDUplink-Downlink todownlinkuplinkconfigu-switchingSub-frame No.rationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
TABLE 2Number of OFDM symbols in DwPTS/UpPTS in respectivespecial sub-frame configurationsDownlink normal cyclicDownlink extended cyclicprefixprefixUpPTSUpPTSSpecialUplinkUplinkUplinkUplinksub-framenormalextendednormalextendedconfigu-cycliccycliccycliccyclicrationDwPTSprefixprefixDwPTSprefixprefix0311311198210931110412322532286997105811———96
For different scenarios, transmission modes of E-PDCCHs can be categorized into localized transmission and distributed transmission in the frequency domain. Typically the localized transmission mode is generally applicable to such a scenario that an evolved Node B (eNB) can obtain precise channel information fed back by a UE and interference from an adjacent cell will not dramatically vary from one sub-frame to another, and in this scenario, according to Channel State Information (CSI) fed back by the UE, the eNB selects consecutive frequency resources with a good quality to transmit E-PDCCHs for the UE and performs a pre-coding/beam-forming process to improve the performance of transmission. If no channel information can be obtained accurately or interference from an adjacent cell dramatically varies from one sub-frame to another and may be unpredictable, then E-PDCCHs need to be transmitted in the distributed transmission mode, that is, they may be transmitted over frequency resources inconsecutive in frequency in order for a gain of frequency diversity. Taking DCI transmitted in an E-PDCCH as an example, the E-PDCCH is transmitted in the localized transmission mode as illustrated in FIG. 1A and in the distributed transmission mode as illustrated in FIG. 1B, where DCI of an E-PDCCH is transmitted over resources in four PRB pairs in both of the transmission modes. In the localized transmission mode, DCI of an E-PDCCH is transmitted in a PRB pair #n, a PRB pair #n+1, a PRB pair #n+2 and a PRB pair #n+3, all of which are consecutive in frequency. In the distributed transmission mode, DCI of an E-PDCCH is transmitted in a PRB pair#A, a PRB pair#B, a PRB pair#C and a PRB pair#D inconsecutive in frequency.
As per ongoing discussion about the design of an E-PDCCH, possible designs of an E-PDCCH are as follows.
Firstly, REs in a PRB pair can be divided into a specific number of Enhanced-Control Channel Elements (E-CCEs), where the same number of REs are included in each E-CCE. DCI can be transmitted to the UE by being carried over a physical resource in a unit of E-CCE. This design is generally applicable to the localized transmission mode.
Without any limitation, each E-CCE can be further divided into a specific number of Enhanced-Resource Element Groups (E-REGs).
Secondly, RE resources in a PRB pair can be divided into a specific number of E-REGs, where the same number of REs are included in each E-REG. Furthermore every K E-REGs can be aggregated into an E-CCE. DCI can be transmitted to the UE by being carried over a physical resource in a unit of E-CCE. This design is generally applicable to the distributed transmission mode.
The number M′ of E-CCEs to carry a piece of DCI of a UE may take the value of 1, 2, 4, 8, 16 or 32 and can be referred to an E-CCE aggregation level.
The REs in the PRB pair above include really available REs for transmission of E-PDCCHs and also possibly a variety of reference signals, e.g., a Common Resource Signal (CRS), a Demodulation Reference Signal (DMRS), a Cell-Specific Reference Signal (CSI-RS), zero-power CSI-RS, possibly a GP in a special sub-frame in the TDD system, etc.
Accordingly with a PRB pair divided into a specific number of E-CCEs/E-REGs as described above, the numbers of REs really available in the respective E-CCE for transmission of E-PDCCHs may significantly differ, even possibly by a factor of two, from one system configuration to another or from one sub-frame to another due to different reference signals configured in the system.
In the current state of art, the UE always performs with its blind detection capability blind detection at the E-CCE aggregation levels of 1, 2, 4 and 8. However an E-PDCCH may not be transmitted at a low E-CCE aggregation level, so it will be inadvisable for the UE to perform blind detection at such a low E-CCE aggregation level, which would otherwise result in an unnecessary energy overhead of the UE in the course of blind E-PDCCH detection.